The simplest active cancellation for phenomena is an analog negative feedback system, also referred to as "a virtual earth" system. In such systems, phenomena is sensed and the sensor input signal is provided to an inverting amplifier. A cancellation signal is sent to an actuator, which provides cancellation phenomena into the area in which the phenomena is to be cancelled.
A problem with analog feedback systems is that they become unstable and produce destructive positive feedback due to phase shifts. These phase shifts are typically due to delays such as that resulting from the distance between the sensor and the actuator, and also by echos. The phase shifts vary by frequency and have not been amenable to a solution in the analog feedback systems except over a very narrow range of frequencies or in a very confined environment, such as a headphone.
Digital cancellation systems have been provided for cancelling repetitive noise. One such system has been described in "Adaptive Noise Cancelling Principals and Applications", by Widrow et al. in Proceedings IEEE Vol. 63, No. 12, December, 1975, which describes two forms of active adaptive cancellers. The first, illustrated in FIG. 5 of that paper uses a multi-tap adaptive FIR filter with a reference signal correlated with the noise to be cancelled. The reference signal is required to be within 90.degree. in phase of the error signal. Consequently, the reference signal used by the adapter itself often requires filtering. This approach is referred to as the "filtered-x algorithm". This approach requires two sensors and does not account for variable delays in the system.
The second form described by Widrow et al. is illustrated in FIG. 6 of that paper, and provides a single frequency notch filter and requires only two single tap filters. Again, a reference signal correlated with the noise is used and is phase shift 90.degree. for one of the filters. This approach cannot operate without the reference signal and does not account for variable delays in the system.
Therefore, there is a need for an active cancellation system for repetitive or non-repetitive phenomena which uses only a single sensor, solves the instabilities and extends the frequency range over that of the conventional analog virtual earth systems, accounts for environmentally produced delays, and is adaptable to maintain an appropriate phase relationship without the use of external reference signals.